1. Field of the Invention
The present invention relates to optical recording medium driving apparatuses and, more particularly, to an optical recording medium driving apparatus capable of accurately detecting the amount of radial movement of an optical head.
2. Description of the Related Art
Various track access methods have been proposed in an optical disc driving apparatus. One of such methods is a track count method. In this method, the number of pulses of a track crossing signal detected by an optical head is counted, the present position of the optical head is detected in accordance with the number of crossed tracks, and the optical head is moved to a target track by a linear motor or the like.
A conventional track count method will now be described. As shown in FIG. 8(a), guide grooves 71, 71 are provided with predetermined spacings on a surface of an optical disc 70, and tracks 72 are formed between adjacent guide grooves 71, 71.
In track access, a light beam 73 radially moves while crossing the tracks 72, 72 along, for example, an indicated arrow 74. The light beam 73 moves outward in the optical disc 70 between A and B in the figure, while the light beam 73 moves inward between B and C. The light beam 73 actually moves orthogonally or approximately orthogonally to the tracks 72, 72 in track access; however, since the optical disc 70 normally rotates also in track access, the trace of the light beam 73 on the optical disc 70 obliquely crosses the tracks 72, 72.
FIG. 8(b) shows the transition of a track error signal 75 when the light beam 73 moves along the indicated arrow 74. FIG. 8(c) shows the transition of a total signal 76 when the light beam 73 moves along the indicated arrow 74. The track error signal 75 is at a zero level in a central portion of the tracks 72 in its width direction, while the total signal 76 is at a maximum level in the central portion of the tracks 72 in its width direction.
The track error signal 75 is a difference signal of output signals of respective light receiving portions in, for example, a bipartite light detector (not shown), while the total signal 76 is a sum signal of the output signals of the respective light receiving portions in the bipartite light detector.
FIG. 8(d) shows a binarized track error signal 77 generated by binarization of the track error signal 75. FIG. 8(e) shows a land/groove determining signal 79 which is obtained by comparing the total signal 76 with a predetermined slice level 78 (FIG. 8(c)) by a comparator not shown and binarizing the compared signal. A low level of this land/groove determining signal 79 corresponds to the guide groove 71 (groove), while a high level thereof corresponds to the track 72 (land).
FIG. 8(f) shows a direction signal 80 obtained by latching the level of the land/groove determining signal 79 at the time of rising of the binarized track error signal 77. The direction signal 80 attains a low level when the light beam 73 is moving outward in the optical disc 70, while the direction signal 80 attains a high level when the light beam 73 is moving inward.
An edge detection signal 81 shown in FIG. 8(g) is pulses output for a predetermined time period after the rising time of the binarized track error signal 77. The edge detection signal 81 corresponds to timing at which the light beam 73 crosses the guide grooves 71 when the light beam is moving outward in the optical disc 70, while the signal 81 corresponds to timing at which the light beam 73 crosses the tracks 72 when the light beam is moving inward in the optical disc 70.
An up signal 82 of FIG. 8(h) and a down signal 83 of FIG. 8(i) are selected from the edge detection signal 81 in accordance with a logic level of the direction signal 80 (the up signal 82 is generated from the edge detection signal 81 when the direction signal 80 is at a low level, while the down signal 83 is generated when the direction signal 80 is at a high level.) The number of pulses of the up signal 82 corresponds to the number which the light beam 73 crosses the tracks 72 outward in the optical disc 70, while the number of pulses of the down signal 83 corresponds to the number which the light beam 73 crosses the tracks 72 inward in the optical disc 70.
Thus, if the up signal 82 and the down signal 83 are counted by an up-down counter not shown, it is then possible to detect the amount of radial movement of the optical head of the optical disc 70.
There is a case, however, where the amount of radial movement of the optical head cannot accurately be detected in the conventional optical disc driving apparatus. Such a case will now be described.
FIG. 9 shows the transition of various types of signals in FIG. 8 when the light beam 73 crosses an ID portion 84 which is recorded in advance by phase pits in advance and represents a track number and a sector number (various pits are not shown and the overall area is shown by hatching for facilitating the description).
In the optical disc 70, when the light beam 73 passes the ID portion 84 previously recorded with phase pits as indicated by the arrow 74, as shown in FIG. 9(a), the track error signal 75 and the total signal 76 are subjected to modulation by phase pits and their waveforms are made irregular in periods 75a and 76a that the light beam 73 passes the ID portion 84, as shown in FIG. 9(b) and (c), respectively. Thus, when the light beam 73 crosses the ID portion 84, the track error signal 75 and the total signal 76 no longer correspond to timing at which the light beam 73 crosses the tracks. In order to eliminate this influence by phase pits, the track error signal 75 and the total signal 76 have their high pass band components, corresponding to phase pits, removed by a low pass filter not shown.
FIG. 9(d) and (e) shows a track error signal 85 and a total signal 86 that have passed the low pass filter, respectively. When the light beam 73 passes the ID portion 84, the track error signal 85 and the total signal 86 are of such waveforms as shown in periods 85a and 86a, respectively, and their high frequency components by phase pits are removed.
In this case, an amplitude merely becomes smaller in the period 85a, and a binarized track error signal 77 (FIG. 9(f)) obtained by being compared with a zero level corresponds to timing at which the light beam 73 crosses the tracks. However, the level of the signal becomes lower than a predetermined slice level 78 in the period 86a. Then, with respect to the land/groove determining signal 79 obtained by comparing the total signal 86 with the slice level 78 by a comparator not shown, a portion which attains a high level as shown by dotted lines in a case where there is no ID portion 84 attains a low level, as shown by solid lines in a period 79a corresponding to the ID portion 84 shown in FIG. 9(g), also during a period that the light beam passes a radial position corresponding to tracks 72.
Thus, the level of the direction signal 80 of FIG. 9(h) changes accurately between a low level and a high level in a period 80a in accordance with the direction of movement of the light beam 73 as shown by dotted lines when there is no ID portion 84. When there is the ID portion 84, however, the direction signal 80 remains at a low level during a period that the light beam 73 moves inward as shown by solid lines in the period 80a.
Therefore, no down signal is generated in a period 83a (FIG. 9(k)) in which the down signal 83 is inherently to be generated, and the up signal 82 is erroneously generated in a period 82a of FIG. 9(j). As a result, there occurs a count error in an up-down counter and hence, an accurate detection of the position of the optical head is not available.